1. Field of the Invention
This invention relates to a system and method for examining tissue of an object to be investigated and is especially directed to in vivo measurement of the sound velocity of the tissue. Through such measurement, the user of the system and method can determine the nature of the tissue and often perform medical or scientific diagnosis.
2. Description of the Prior Art
Nishimura N. et al., "Measurement of Sound Velocity as a Diagnostic Tool Diffuse Liver Diseases," Japanese Journal of Medical Ultrasonics, 44-B-5, May 1984 shows a technique of measuring sound velocity of liver with two single transducers disposed at both ends of a linear array transducer. These two single transducers are disposed so that their beams cross within the liver. An ultrasonic pulse and its echo propagate from one single transducer through that cross point and to the other transducer. The propagation time taken by the pulse to travel that path is measured. The average sound velocity through the propagation path is computed from the propagation time, the distance between the two transducers, and the angles of propagation defined by the placement of the transducers. In this past technique, however, it is difficult to examine an internal tissue. Such an internal tissue, such as a liver, lies under a fat or muscle layer near the skin surface. The sound velocity is slower in the fat layer and faster in the muscle layer than it is in the liver. Therefore the propagation time taken by the pulse to travel that path depends on the thickness of the fat or muscle layer. This fat or muscle layer sometimes has a uniform thickness and sometimes has a nonuniform one. Because of this problem, it is difficult to obtain an accurate measurement of the sound velocity of the internal tissue.